1. Field of the Invention
This invention relates to a method and an apparatus used for joining a new electrode to an old and consumed electrode fitted to an electric furnace, and in particular to a method for aligning both axes of the new and the consumed electrodes, a method for controlling an electric motor in order to tighten both the electrodes with each other, and an apparatus for lifting the new electrode and rotating the new electrode so as to screw and tighten both the electrodes with each other.
2. Description of the Prior Art
In an electric furnace used for manufacturing, for example, steel, ferro alloy and so on, by generating arc between electrodes and materials charged in a hearth, the electrodes are consumed according to continuation of the operation. Therefore, in the electric furnace of this kind, the electrodes supported with electrode holders of the electric furnace are so designed as to be fed into the furnace according to the consumption of the electrodes through electrode holes disposed in a furnace cover, and are extended by adding new electrodes to the old and consumed electrodes.
Heretofore, on the occasion of joining the new electrode to the old and consumed electrode, a new electrode 100 to be added to a consumed electrode 106 is suspended by a crane 102 through a joining apparatus 103 and moved to a position just above the consumed electrode 106 of an electric furnace 104 as shown in FIG. 11. Subsequently, the new electrode 100 is joined to the old and consumed electrode 106 fitted to the electric furnace 104 through a holder arm 105 by rotating the suspended new electrode 100 with an electric motor disposed in the joining apparatus 103 at the same time of lowering the new electrode 100 to screw a screw thread 108 formed on its lower end into a threaded hole 110 formed in the upper end of the consumed electrode 106 of the electric furnace 104.
However, in such the method for joining electrodes 100 and 106, it is difficult to align the axis of the new electrode 100 suspended from the crane 102 with the axis of the consumed electrode 106 of the electric furnace 104 accurately, and there is a problem since it requires much labor and time to align the axes of both the electrodes 100 and 106.
Additionally, at the time of screwing the screw thread 108 of the new electrode 100 into the treaded hole 110 of the consumed electrode 106 by rotating the new electrode 100 through the electric motor of the joining apparatus 103, it is difficult to make the final tightening torque applied on the threaded portions 108 and 109 regular always because of subtle differences in size of the threaded portions 108 and 109 caused by scattering of manufacturing accuracy. Accordingly, there is the possibility that the threaded portions 108 and 110 are tighten insufficiently or excessively depending on circumstances.
Namely, at the time of screwing the screw thread 108 of the new electrode 100 into the threaded hole 110 of the consumed electrode 106 to tighten both the electrodes 100 and 106 by rotating the new electrode 100 through the electric motor of the joining apparatus 103, the electric motor is put into an overload state when both the electrodes 100 and 106 are tighten in some degree, and is brought to a standstill, that is the motor falls into a stalling state.
Stalling operation means to work the motor in such the overload state, an electric current flowing in the motor is held constant and torque output from the motor shows fixed value in this state.
Generally, if the motor is connected to the source of commercial electric power supply and operated in the stalling state, an overcurrent flows in the motor, the motor overheats and is burned out before long.
Therefore, the electric motor is prevented from the overheating by cutting off the power supply at the same time the motor falls into the stalling state in the conventional method. In such the method, however, it is impossible to apply the tightening torque required for tightening both the electrodes 100 and 106 to the threaded portions 108 and 110.
Consequently, there is another problem in that the threaded portions 108 and 110 generate heat owing to increase of electric resistance therebetween if the threaded portions 108 and 110 are tightened insufficiently, and threads of the threaded positions 108 and 110 may be broken by tightening them excessively, thereby causing disconnection of the electrodes and deterioration of the lifetime of the electrodes.
Furthermore, although the new electrode 100 to be added to the consumed electrode 106 is generally packed horizontally and stored in a state of being laid down on the floor as shown in FIG. 12, a connector 103 a of the conventional joining apparatus 103 is merely possible to be connected with the new electrode 100 in a standing state. Therefore, it is necessary to set up the electrode 100 laid on the floor and set it in the standing position in order to connect the new electrode 100 to the connector 103a of the joining apparatus 103.
Accordingly, a problem arises in that it requires much time and hard labor to set the electrode 100 to the joining apparatus 103.